Bethesda's gamebryo monstrosity is one of the crappiest engines in recent memory. It was clunky in Morrowind back in 2001 and they've just glued more crap onto it over the years.

On the other hand, ET:QW showed that id's engines are making some pretty massive improvements in performance and visual quality for large levels. And what I've seen so far of idTech6 has been very impressive.